Only return (tw) from tcp_twclose() if reuse is passed, otherwise

return NULL.  In principle this shouldn't change the behavior, but
avoids returning a potentially invalid/inappropriate pointer to
the caller.

Found with:	Coverity Prevent (tm)
Submitted by:	pjd
MFC after:	3 months

sys/netinet/tcp_timer.c

@@ -295,7 +295,7 @@ tcp_timer_2msl_tw(int reuse)
 			continue;
 		INP_LOCK(tw->tw_inpcb);
 		tcp_twclose(tw, reuse);
-		return (tw);
+		return (reuse ? tw : NULL);
 	}
 	return (NULL);
 }